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Gradient corrections for reference dosimetry using Farmer-type ionization chambers in single-layer scanned proton

Hugo Palmans1,2, Joakim Medin3, Petra Trnková4,5

  • 1MedAustron Ion Therapy Center, Wiener Neustadt, Austria.

Medical Physics
|October 28, 2020
PubMed
Summary

Accurate proton beam dosimetry using Farmer-type ionization chambers requires careful consideration of the effective point of measurement. This study quantifies dose gradients and displacement factors, providing recommendations for optimal chamber positioning and corrections.

Keywords:
Farmesdepth dose gradientdisplacement correctioneffective point of measurementionization chamberreference dosimetryscanned proton beam

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Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Dosimetry

Background:

  • Accurate dose determination is critical for effective proton therapy.
  • Farmer-type ionization chambers are standard tools for reference dosimetry.
  • Scanned proton beams present unique challenges for dosimetry due to steep dose gradients.

Purpose of the Study:

  • To quantify the local depth dose gradient in scanned proton fields.
  • To determine the displacement correction factor for Farmer-type ionization chambers at shallow depths.
  • To provide recommendations for reference dosimetry in scanned proton beams.

Main Methods:

  • Analysis of integrated radial profiles as a function of depth (IRPDs) from three proton therapy centers.
  • Polynomial fitting of IRPDs to derive local depth dose gradients.
  • Literature review to establish the effective point of measurement for Farmer-type ionization chambers.

Main Results:

  • Depth dose gradients vary from 0.1-0.4% per mm to over 3% per mm depending on residual range.
  • The effective point of measurement is located 0.75 times the cavity radius closer to the beam origin than the cavity center.
  • Farmer-type ionization chambers can be used for depths with residual ranges >= 4 cm with specific corrections or positioning.

Conclusions:

  • The effective point of measurement for Farmer-type ionization chambers in proton beams is confirmed.
  • Three dosimetry options are recommended: positioning the effective point, applying displacement correction, or no correction under specific conditions.
  • Options for accurate dosimetry are feasible for residual ranges of at least 4 cm, with stricter requirements for uncorrected measurements.